Golf ball

ABSTRACT

The present invention provides a golf ball composed of a core, an outermost cover layer and one or more intermediate layer therebetween. At least one intermediate layer includes a mixture composed of a base polymer made of, in a specific ratio, (a) a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random copolymer, (b) a thermoplastic elastomer selected from the group consisting of thermoplastic polyester elastomers, thermoplastic block copolymers and thermoplastic polyurethanes, and (c) a thermoplastic block copolymer containing end blocks modified by a functional group having reactivity with an ionomer resin, both end blocks being formed of different comonomers. The mixture also includes (e) an organic acid of 18 to 40 carbons or a derivative thereof, and (f) a metal ion source selected from the group consisting of alkali metals, alkaline earth metals and transition metals. The mixture has an optimized difference between the melt viscosities of (b) and (a) as measured under a shear rate of 6,000 sec −1  and 240° C. in accordance with JIS K-7199. The golf ball of the invention has a flight and control performance acceptable for use by professional golfers and skilled amateurs, and also has an excellent durability to cracking under repeated impact.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.11/783,939 filed on Apr. 13, 2007, now U.S. Pat. No. 7,527,566, thecontents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-layer golf ball having a core,a cover, and at least one intermediate layer position therebetween. Morespecifically, the invention relates to a golf ball which has a flightand control performance acceptable for use by professional golfers andskilled amateurs, and which also has an excellent durability to crackingunder repeated impact.

In addition to having a good rebound, it is important for the enjoymentof the game that a golf ball also provide the player with a good feel onimpact. To impart such a feel, the cover serving as the outermost layeris generally made soft. Art to this effect has been disclosed innumerous patent publications.

For example, one disclosure for ensuring a Shore D hardness of about 50and resilience involves blending a thermoplastic polyester elastomer oran olefinic thermoplastic elastomer into a cover material in which anionomer serves as the base resin.

Yet, such a cover material still leaves something to be desired in termsof resilience. Improvements have thus been disclosed in which behenicacid and calcium hydroxide are added to the cover material so as tocarry out a high degree of neutralization and thereby impart a highresilience. Such art is described in, for example, U.S. Pat. No.6,838,501 (corresponding to JP-A 2002-219195) and U.S. Pat. No.7,091,269 (corresponding to JP-A 2005-013487).

However, in the former, a non-ionomeric material having a high meltviscosity is used in the cover material, making it difficult to achievea thin cover by injection molding. In the latter, high-speed injectionmolding of the cover material causes laminar separation to occur,lowering the durability of the ball.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golfball which has an acceptable flight performance and controllability andwhich, even when a thin intermediate layer is used, has an excellentdurability to cracking on repeated impact.

As a result of extensive investigations, the inventor has found thatgolf balls which contain a thin intermediate layer composed of aspecific novel material have an excellent flight performance andcontrollability, and also have an excellent durability to cracking onrepeated impact.

Thermoplastic polyester elastomers generally have a low melt viscosityand a good resilience, but they lack compatibility with ionomers. Inparticular, when the ionomer is highly neutralized, if the resins have alarge difference in melt viscosity, laminar separation tends to occur.Hence, the present invention, by using an ionomer and a thermoplasticpolyester elastomer together and thus balancing the different meltviscosities of the two resins, creates a resin which resists laminarseparation even when the ionomer is highly neutralized and high-speedinjection molding is carried out. The invention also reflects thefinding that, to be able to achieve a thin intermediate layer having athickness of 1.2 mm or less, when the combined resin is injection-moldedat a high speed, it is important from the standpoint of durability tobalance the melt viscosities of the differing constituent resins.

Accordingly, the invention provides the following golf balls.

-   [1] A golf ball comprising a core, an outermost cover layer and one    or more intermediate layer therebetween, wherein at least one    intermediate layer includes a mixture comprising:    -   100 parts by weight of a base polymer composed of        -   (a) a metal ion neutralization product of an            olefin-unsaturated carboxylic acid random copolymer and/or a            metal ion neutralization product of an olefin-unsaturated            carboxylic acid-unsaturated carboxylic acid ester random            copolymer,        -   (b) a thermoplastic elastomer selected from the group            consisting of thermoplastic polyester elastomers,            thermoplastic block copolymers and thermoplastic            polyurethanes, and        -   (c) a thermoplastic block copolymer containing end blocks            modified by a functional group having reactivity with an            ionomer resin, both end blocks being formed of different            comonomers    -    in a weight ratio a/b/c=50 to 80/50 to 20/4 to 20,    -   (e) 5 to 150 parts by weight of an organic acid of 18 to 40        carbons or a derivative thereof, and    -   (f) 0.1 to 10 parts by weight of a metal ion source selected        from the group consisting of alkali metals, alkaline earth        metals and transition metals:        which mixture satisfies the condition 0≦(b)−(a)≦25, where        (b)−(a) is the difference between the melt viscosity of        component (b) and the malt viscosity of component (a) as        measured at a shear rate of 6,000 sec⁻¹ and 240° C. in        accordance with JIS K-7199.-   [2] The golf ball of [1], wherein the mixture additionally comprises    10 to 30 parts by weight of an inorganic filler which is    non-reactive with the ionomer resin (d) per 100 parts by weight of    the base polymer.-   [3] The golf ball of [2], wherein component (b) is a thermoplastic    polyester elastomer-   [4] The golf ball of [1], wherein the intermediate layer comprised    of above components (a) to (f) has a thickness of at most 1.2 mm.

BRIEF DESCRIPTION OF THE DIAGRAM

FIG. 1 is a schematic cross-sectional view of a multi-piece solid golfball (four-layer structure) according to one embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The core of the inventive golf ball may be a rubber core obtained bymolding and vulcanizing a rubber composition containing primarily acommonly used base rubber. Specifically, the core is made of a moldedand vulcanized material prepared from a rubber composition containingthe following ingredients A to E;

(A) base rubber 100 parts by weight (B) unsaturated carboxylic acid 10to 50 parts by weight and/or metal salt thereof (C) organosulfurcompound 0.1 to 5 parts by weight (D) inorganic filler 5 to 80 parts byweight, and (E) organic peroxide 0.1 to 3 parts by weight.Other ingredients, such as antioxidants and elemental sulfur, may beadded to the rubber composition an needed in a combined amount of from0.05 to 3 parts by weight per 100 parts by weight of component A.

The base rubber (A) of this rubber composition, while not subject to anyspecific limitation, is typically a general-purpose synthetic rubberused in core formulations, and preferably one in which polybutadieneserves as the primary component. “Primary component” here signifies thatthe polybutadiene accounts for at least 50 wt %, preferably at least 70wt %, and most preferably 100 wt %, of the base rubber.

The polybutadiene is product synthesized using a rare earth catalyst ora group VIII catalyst. The synthesis catalyst is preferably a rare-earthcatalyst. Known rare-earth catalysts may be used for this purpose, suchas catalysts made up of a combination of a lanthanide series rare-earthcompound, an organoaluminum compound, an alumoxane, a halogen-bearingcompound and an optional Lewis base, or containing a metallocenecomplex.

Sulfur may be added to the polybutadiene to increase the core hardnessdistribution. Such sulfur is exemplified by powdered sulfur, a specificexample of which is the dispersible sulfur manufactured by TsurumiChemical Industry Co., Ltd. under the trade name “Sulfur Z.”

Next, the properties of the solid core used in the inventive golf ballare described.

It is recommended that the solid core have a diameter of generally atleast 34.0 mm, preferably at least 35.0 mm, more preferably at least35.5 mm, and most preferably at least 36.3 mm, but generally not morethan 41.0 mm, preferably not more than 39.0 mm, more preferably than38.5 mm, and most preferably not more than 38.2 mm.

The hardness difference calculated by subtracting the Shore D hardnessat the center of the solid core from the Shore D hardness at the surfaceof the core is adjusted to generally at least 10, preferably at least12, more preferably at least 13, and most preferably at least 14, butnot more than 40, preferably not more than 36, more preferably not morethan 31, and most preferably not more than 25. Optimizing this hardnessdifference lowers the spin rate on full shots taken with a driver,enabling the distance traveled by the ball to be increased.

The surface hardness of the core is the value obtained by directlymeasuring the hardness at the core surface with a Shore D durometer. Thecore center hardness is the value obtained by cutting the core in halfand measuring the hardness at the center of the resulting section with aShore D durometer. Each of these hardness values is the average of themeasurements obtained at two points on ten cores.

The core may be obtained by vulcanizing and curing the above-describedrubber composition using a method similar to that employed with knownrubber compositions for golf balls. For example, vulcanization may becarried out at a temperature of between 100 and 200° C. for a period ofbetween 10 and 40 minutes.

In the practice of the invention, at least one intermediate layer isinterposed between the core and the cover serving as the outermostlayer. For example, FIG. 1 shows a multi-piece solid golf ball G whichis composed of four layers: a core 1 and an outermost cover layer 4having a plurality of dimples D formed thereon, between which areprovided a first intermediate layer 2 and a second intermediate layer 3that encloses the first intermediate layer 2.

At least one of the intermediate layers in the inventive golf ball isformed of a resin composition consisting primarily of:

-   100 parts by weight of a base polymer composed of    -   (a) a metal ion neutralization product of an olefin-unsaturated        carboxylic acid random copolymer and/or a metal ion        neutralization product of an olefin-unsaturated carboxylic        acid-unsaturated carboxylic acid aster random copolymer,    -   (b) a thermoplastic elastomer selected from the group consisting        of thermoplastic polyester elastomers, thermoplastic block        copolymers and thermoplastic polyurethanes, and    -   (c) a thermoplastic block copolymer containing end blocks        modified by a functional group having reactivity with an ionomer        resin, both end blocks being formed of different comonomers-    in a weight ratio a/b/c=50 to 80/50 to 20/4 to 20,    -   (e) 5 to 150 parts by weight of an organic acid of 18 to 40        carbons or a derivative thereof, and    -   (f) 0.1 to 10 parts by weight of a metal ion source selected        from the group consisting of alkali metals, alkaline earth        metals and transition metals.        Component (a)

The olefin in component (a) has a number of carbons which is generallyat least 2 but not more than 8, and preferably not more than 6. Specificexamples include ethylene, propylene, butane, pentene, hexene, hepteneand octene. Ethylene is especially preferred.

Examples of the unsaturated carboxylic acid in component (a) includeacrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylicacid and methacrylic acid are especially preferred.

The unsaturated carboxylic acid ester in component (a) is preferably alower alkyl ester of the above unsaturated carboxylic acid. Specificexamples include methyl methacrylate, ethyl methacrylate, propylmethacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate,propyl acrylate and butyl acrylate. Butyl acrylate (n-butyl acrylate,i-butyl acrylate) is especially preferred.

In the practice of the invention, above component (a) may include artolefin-unsaturated carboxylic acid random copolymer and/or anolefin-unsaturated carboxylic acid-unsaturated carboxylic acid asterrandom copolymer (referred to below as “the random copolymers”). Theserandom copolymers may be respectively obtained by preparing each of theabove-described materials and using a known method to carry out randomcopolymerization.

It is preferable for the above random copolymers to have controlledunsaturated carboxylic acid contents (acid contents). Here, it isrecommended that the content of unsaturated carboxylic acid present inthe random copolymer of component (a) be generally at least 4 wt %,preferably at least 6 wt %, more preferably at least 8 wt %, and evenmore preferably at least 10 wt %, but not more than 30 wt %, preferablynot more than 20 wt %, more preferably not more than 18 wt %, and evenmore preferably not more than 15 wt %.

The metal ion neutralization product of the olefin-unsaturatedcarboxylic acid random copolymer and the metal ion neutralizationproduct of the olefin-unsaturated carboxylic acid-unsaturated carboxylicacid ester random copolymer in the invention (referred to below as “themetal ion neutralization products”) may be obtained by neutralizing someof the acid groups on the above random copolymers with metal ions.

Illustrative examples of metal ions that may be used here forneutralizing acid groups include Na⁺, K⁺, Li⁺, Zn⁺⁺, Mg⁺⁺ and Ca⁺⁺. Theuse of Na⁺ is preferred.

To obtain the metal ion neutralization product of a random copolymerused in the invention, it suffices to neutralize the above-describedrandom copolymer with the above-mentioned metal ions. For example, usemay be made of a method that involves neutralization using a compoundsuch as a formats, acetate, nitrate, carbonate, bicarbonate, oxide,hydroxide or alkoxide of the above metal ions. To further facilitatemolding at the time of resin mixture, the degree of neutralization ofthe random copolymer by these metal ions is generally at least 10%, andpreferably at least 20%, but generally not more than 80%, and preferablynot more than 70%.

Component (b)

Thermoplastic polyester elastomers that may be used as component (b) arecomposed primarily of hard segments which are high-melting crystallinepolymer segments made up of crystalline aromatic polyester units, andsoft segments which are low-melting polymer segments made up ofaliphatic polyether units and/or aliphatic polyester units.

Preferred examples of the high-melting crystalline polymer serving asthe hard segments include polybutylene terephthalates derived fromterephthalic acid and/or dimethyl terephthalate in combination with1,4-butanediol. Other illustrative examples include polyesters derivedfrom a dicarboxylic acid component such as isophthalic acid, phthalicacid, naphthalene-2,6-dicarboxylic acid, naphthalene-2,7-dicarboxylicacid, diphenyl-4,4′-dicarboxylic acid, diphenoxyethanedicarboxylic acid,5-sulfoisophthalic acid or ester-forming derivatives thereof incombination with a diol having a molecular weight of up to 300, such asan aliphatic diol (e.g., ethylene glycol, trimethylene glycol,pentamethylene glycol, hexamethylene glycol, neopentyl glycol,decamethylene glycol), an alicyclic diol (e.g.,1,4-cyclohexanedimethanol, tricyclodecanedimethylol), or an aromaticdiol (e.g., xylylene glycol, bis(p-hydroxy)diphenyl,bis(p-hydroxyphenyl)propane, 2,2-bis[4-(2-hydroxyethoxy)-phenyl]propane,bis[4-(2-hydroxy)phenyl]sulfone,1,1-bis[4-(2-hydroxyethoxy)phenyl]cyclohexane,4,4′-dihydroxy-p-terphenyl, 4,4′-dihydroxy-p-quarterphenyl). Use canalso be made of copolymeric polyesters prepared from two or more ofthese dicarboxylic acid components and diol components. In addition,polycarboxylic acid components, polyoxy acid components and polyhydroxycomponents having a functionality of three or more may be copolymerizedin component (b) within a range of up to 5 mol %.

The low-melting polymer serving as the soft segments are low-meltingpolymer segments composed of aliphatic polyether units and/or aliphaticpolyester units.

Illustrative examples of aliphatic polyether units include poly(ethyleneoxide) glycol, poly(propylene oxide) glycol, poly(tetramethylene oxide)glycol, poly(hexamethylene oxide) glycol, copolymers of ethylene oxideand propylene oxide, ethylene oxide addition polymers of polypropyleneoxide) glycols, and copolymers of ethylene oxide and tetrahydrofuran.Illustrative examples of aliphatic polyester units includepoly(ε-caprolactone), polyenantholactone, polycaprylolactone,poly(butylene adipate) and poly(ethylene adipate). Of the abovepolymers, in terms of the resilience characteristics of the resultingpolyester block copolymer, poly(tetramethylene oxide) glycol, ethyleneoxide addition polymers of polypropylene oxide) glycol,poly(ε-caprolactone), poly(butylene adipate) and poly(ethylene adipate)are preferred. Poly(tetramethylene oxide) glycol is especiallypreferred.

The low-melting polymer segments have a number-average molecular weightin the copolymerized state of preferably about 300 to about 6,000.

Letting the combined amount of high-melting crystalline polymer segmentsand low-melting polymer segments which are copolymerized to form thethermoplastic polyester elastomer be 100 wt %, it is advantageous forthe polyester elastomer to include at least 15 wt %, and preferably atleast 50 wt %, but not more than 90 wt %, of the low-melting polymersegments. At a proportion of low-malting polymer segments higher thanthe above range, adequate melt characteristics suitable for injectionmolding may not be attainable, which can make it difficult to achieveuniform mixture during melt blending with the other components. On theother hand, if the proportion is too low, sufficient flexibility andresilience may not be achieved.

The above-described thermoplastic polyester elastomer is a copolymercomposed primarily of the foregoing high-melting crystalline polymersegments and low-melting polymer segments. The thermoplastic polyesterelastomer may be prepared by a known method without particularlimitation. Exemplary methods of preparation include methods (i) to (v)below, any of which may be suitably used.

-   (i) A method in which a lower alcohol diester of a dicarboxylic    acid, an excess amount of low-molecular-weight glycol, and the    low-melting polymer segment component are subjected to    transesterification in the presence of a catalyst, and the resulting    reaction products are polycondensed.-   (ii) A method in which a dicarboxylic acid, an excess amount of a    glycol and the low-melting polymer segment component are subjected    to esterification in the presence of a catalyst, and the resulting    reaction products are polycondensed.-   (iii) A method in which first the high-melting crystalline segments    are prepared, then the low-melting segments are added thereto and a    transesterification reaction is carried out to effect randomization.-   (iv) A method in which the high-melting crystalline segments and the    low-melting polymer segments are joined together using a chain    linking agent.-   (v) In cases where poly(ε-caprolactone) is used as the low-melting    polymer segments, a method in which the high-melting crystalline    segments are subjected to an addition reaction with ε-caprolactone    monomer.

It is recommended that the above-described thermoplastic polyesterelastomer have a hardness, as measured in accordance with ASTM D-2240(Shore D hardness), of generally at least 10, and preferably at least20, but not more than 50, and preferably not more than 40. It ispreferable for the above thermoplastic polyester elastomer to be softerthan the component (a) mixture.

Moreover, it is advantageous for the above thermoplastic polyesterelastomer to exhibit a high rebound resilience, as measured inaccordance with British Standard 903 (BS 903), of generally at least40%, and preferably at least 50%, but not more than 90%. If thecomponent has too low a rebound resilience, molded materials obtainedfrom the resin composition of the invention will have a low resilience,which may diminish the flight performance of golf balls made therewith.

It is desirable for the above thermoplastic polyester elastomer to havea flexural rigidity, as measured in accordance with JIS K-7106, which isrelatively low, with a value of generally at least 5 MPa, preferably atleast 10 MPa, and more preferably at least 15 MPa, but not more than 250MPa, preferably not more than 200 MPa, and even more preferably not morethan 150 MPa. If the flexural rigidity is too high, molded materialsobtained from the resin composition of the invention will be too rigid,which may worsen the feel on impact and the durability of golf ballsmade with such molded materials.

Thermoplastic block copolymers that may be used as component (b) includethose in which the hard segments are made of crystalline polyethyleneblocks (C) and/or crystalline polystyrene blocks (S), and the softsegments are made of polybutadiene blocks (B), polyisoprene blocks (I),blocks of a relatively random copolymer of ethylene and butylene (EB) orblocks of a relatively random copolymer of ethylene and propylene (EP),preferably blocks of a relatively random copolymer of ethylene andbutylene (EB) or blocks of a relatively random copolymer of ethylene andpropylene (EP), and most preferably blocks of a relatively randomcopolymer of ethylene and butylene (EB).

Illustrative examples of such thermoplastic block copolymers includeS-EB-S, S-B-S, S-I-S, S-EB, S-EB-S-ED, S-EP-S, S-EB-C, S-B-C, S-I-C,S-EP-C, C-EB-C, C-B-C, C-I-C, C-EB, C-ERB-C-B and C-EP-C. Includingcrystalline polyethylene blocks (C) as the hard segments is advantageousfrom the standpoint of resilience. The use of S-EP-C is preferred, andthe use of C-EB-C is especially preferred.

If the thermoplastic black copolymer is a C-EB-C or S-EB-C type blockcopolymer, this may be obtained by hydrogenating butadiene or astyrene-butadiene copolymer.

A polybutadiene in which bonding within the butadiene structure ischaracterized by the presence of block-like 1,4-polymer regions having a1,4-bond content of at least 95 wt %, and in which the butadienestructure as a whole has a 1,4-bond content of at least 50 wt %, andpreferably at least 80 wt %, may be suitably used here as thepolybutadiene or styrene-butadiene copolymer subjected to hydrogenation.

The degree of hydrogenation (conversion of double bonds in thepolybutadiene or styrene-butadiene copolymer to saturated bonds) in thehydrogenate is preferably from 60 to 100%, and more preferably from 90to 100%. Too low a degree of hydrogenation may give rise to undesirableeffects such as gelation in the blending step with other components suchas an ionomer resin and, when the golf ball is formed, may compromisethe weather resistance and durability to impact of the cover.

In the thermoplastic block copolymer, the content of the hard segmentsis preferably from 10 to 50 wt %. If the content of hard segments to toohigh, the intermediate layer may lack sufficient flexibility, making itdifficult to effectively achieve the objects of the invention. On theother hand, if the content of hard segments is too low, the blend mayhave a poor moldability.

The thermoplastic block copolymer has a number-average molecular weightof preferably from 30,000 to 800,000. The thermoplastic block copolymerhas a melt index at 230° C. of preferably 0.5 to 15 g/10 min, and morepreferably 1 to 7 g/10 min. Outside of this range, problems such as weldlines, sink marks and short shots may arise during injection molding.

Thermoplastic polyurethane elastomers that may be used as component (b)preferably have a morphology composed of, in particular, ahigh-molecular-weight polyol compound as the soft segments, amonomolecular chain extender, and a diisocyanate as the hard segments.

The high-molecular-weight polyol compound is not subject to anyparticular limitation and may be, for example, a polyester polyol, apolyol polyol, a polyether polyol, a copolyester polyol or apolycarbonate polyol. Exemplary polyester polyols includepolycaprolactone glycol, poly(ethylene-1,4-adipate) glycol andpoly(butylene-1,4-adipate) glycol; exemplary copolyester polyols includepoly(diethylene glycol adipate) glycol; exemplary polycarbonate polyolsinclude (hexanediol-1,6-carbonate) glycol; and exemplary polyetherpolyols include polyoxytetramethylene glycol.

These high-molecular-weight polyol compounds have a number-averagemolecular weight of about 600 to about 5,000, and preferably about 1,000to about 3,000.

An aliphatic or aromatic diisocyanate may be suitably used as thediisocyanate. Illustrative examples include hexamethylene diisocyanate(HDI), 2,2,4- and 2,4,4-trimethylhexamethylene diisocyanate (TMDI),lysine diisocyanate (LDI), tolylene diisocyanate (TDI) anddiphenylmethane diisocyanate (MDI). For a good compatibility whenblending with other resins, the use of hexamethylene diisocyanate (HDI)or diphenylmethane diisocyanate (MDI) is preferred.

The monomolecular chain extender, which is not subject to any particularlimitation, may be an ordinary polyhydric alcohol or polyamine. Specificexamples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-propyleneglycol, 1,6-hexylene glycol, 1,3-butylene glycol,dicyclohexylmethylmethanediamine (hydrogenated MDI) andisophoronediamine (IPDA).

The above thermoplastic polyurethane elastomer has a JIS A hardness oftypically from 70 to 100, preferably from 80 to 98, more preferably from90 to 99, and even more preferably from 95 to 98. At a JIS A hardnessbelow 70, the ball may take on excessive spin when hit with a driver,shortening the distance traveled. No particular limitation is imposed onthe specific gravity of the thermoplastic polyurethane elastomer, solong as it is suitably adjusted within a range where the objects of theinvention are attainable. The specific gravity is preferably between 1.0and 1.3, and more preferably between 1.1 and 1.25.

A commercial product may be used as the above-described thermoplasticpolyurethane elastomer. Illustrative examples include Pandex TR-3080,Pandex T-7298, Pandex EX-7895, Pandex T-7890 and Pandex T-8198 (allmanufactured by DIC Bayer Polymer, Ltd.).

Melt Viscosities of Components (a) & (b) and the Difference Therebetween

It is critical that the difference between the melt viscosity ofcomponent (b) and the melt viscosity of component (a), as measured at ashear rate of 6,000 sec⁻¹ and 240° C. in accordance with JIS K-7199,satisfy the condition 0≦(b)−(a)≦25. The upper limit value is 25 or less,and preferably 20 or less, more preferably 15 or less. By specifying thedifference between the melt viscosity of component (b) and the meltviscosity of component (a) to the above-described range, laminarseparation during high-speed injection molding can be suppressed, makingit possible to increase the durability of the golf ball to cracking.Conversely, outside of the above numerical range, the compatibilitybetween components (a) and (b) worsens, resulting in laminar separationof the resins, particularly after high-speed injection molding, thusdramatically lowering the durability of the ball to cracking. Withregard to the range of the melt viscosity of component (a), which is notsubject to any particular limitation, the lower limit value ispreferably at least 55, more preferably at least 60, and the upper limitvalue is preferably not more than 100, more preferably not more than 95,even more preferably not more than 90, and most preferably not more than85. On the other hand, with regard to the range of the melt viscosity ofcomponent (b), which is not subject to any particular limitation, thelower limit value is preferably at least 55, more preferably at least60, and the upper limit value is preferably not more than 100, morepreferably not more than 95, even more preferably not more than 90, andmost preferably not more than 85. These melt viscosities of component(a) & (b) are set in the above-described range, and if the meltviscosity is too high, the load to the used machine may be too much whenkneading the components. In reverse, the melt viscosity is too low, anoptimum shearing stress may not be applied in kneading so that themixing reaction may not be conducted properly, which will beinconvenient.

Component (c)

Next, the thermoplastic block copolymer which contains end blocksmodified by a functional group having reactivity with an ionomer resin,wherein both end blocks are formed of different comonomers, and servesas component (c) is described.

The thermoplastic block copolymer serving as the base of component (c)is exemplified by block copolymers of the following types: H₁-S₁,H₁-S₁-H₁-S₁, H₁-(S₁-H₁)_(n)-S₁ and H₁-S₁-H₂ (wherein H₁ and H₂ are hardsegments, and S₁ is a soft segment). In particular, the use of an H₁-S₁type di-block copolymer or an H₁-S₁H₂ type tri-block copolymer ispreferred. The use of an H₁-S₁-H₁ type tri-block copolymer is morepreferred. Compared with the use of other block copolymers, graftcopolymers and random copolymers, the compatibility can be markedlyimproved.

Hard segments that may be used in component (c) are exemplified bycrystalline olefin blocks, aromatic vinyl compound blocks, polyesterblocks and polyamide blocks. Effective improvement in the compatibilityis achieved particularly with the use of preferably crystalline olefinblocks, aromatic vinyl compound blocks or polyester blocks, and morepreferably crystalline olefin blocks or aromatic vinyl compound blocks.Crystalline olefin blocks are exemplified by crystalline ethylene blocks(C) and crystalline propylene blocks. The use of crystalline ethyleneblocks is especially preferred. Preferred use may be made of styreneblocks (S) as the aromatic vinyl compound blocks, of polytetramethyleneterephthalate blocks (PBT) as the polyester blocks, and of nylon blocksas the polyamide blocks.

Soft segments that may be used include polybutadiene blocks (B),polyisoprene blocks (I), blocks of a relatively random copolymer ofethylene and butylene (EB), and blocks of a relatively random copolymerof ethylene and propylene (EP). The use of blocks of a relatively randomcopolymer of ethylene and butylene (EB) or blocks of a relatively randomcopolymer of ethylene and propylene (EP) is preferred, and the use ofblocks of a relatively random copolymer of ethylene and butylene (EB) isespecially preferred.

The thermoplastic block copolymer used as component (c) is made ofblocks, with the blocks at either end of the copolymer being formed ofdifferent comonomers. Illustrative examples of such thermoplastic blockcopolymers include S-EB-C, S-B-C, S-I-C, S-EB, S-EB-S-EB, S-EP-C,PBT-S-EB and PBT-S-EB-C. To more effectively improve the compatibilityof the ionomer resin and the thermoplastic elastomer, it is preferableto use S-ED-C or PBT-S-EB, and more preferable to use S-EB-C.

If the thermoplastic block copolymer is an S-EB-C type block copolymer,it can be obtained by hydrogenating a styrene-butadiene copolymer.

A polybutadiene in which bonding within the butadiene structure ischaracterized by the presence of block-like 1,4-polymer regions having a1,4-bond content of at least 95 wt %, and in which the butadienestructure as a whole has a 1,4-bond content of at least 50 wt %, andpreferably at least 80 wt %, may be suitably used here as thepolybutadiene or styrene-butadiene copolymer in hydrogenation.

The degree of hydrogenation (conversion of double bonds in thepolybutadiene or styrene-butadiene copolymer to saturated bonds) in thehydrogenate of the styrene-butadiene copolymer is preferably 60 to 100%,and more preferably 90 to 100%. Too low a degree of hydrogenation maygive rise to undesirable effects such as gelation in the blending stepwith other components such as an ionomer resin and, when the golf ballis formed, may compromise the weather resistance and the durability ofthe ball to impact.

In the above block copolymer having crystalline olefin blocks, thecontent of the hard segments is preferably from 10 to 50 wt %. If thecontent of hard segments is too high, the intermediate layer may lacksufficient flexibility, making it difficult to effectively achieve theobjects of the invention. On the other hand, if the content of hardsegments is too low, the resulting blend may have a poor moldability.

The block copolymer having such crystalline olefin blocks has anumber-average molecular weight of preferably from 30,000 to 800,000.

In the practice of the invention, only the end blocks on thethermoplastic block copolymer are modified with functional groups. Thecompatibility can be very effectively improved in this way compared withwhen only intermediate blocks are modified, when both intermediateblocks and blocks are modified, or when modification is carried outalong the entire molecule, as in random copolymers.

The end block modifying method is preferably a method in which only theends of molecule are modified. This has the advantage that morefunctional groups than necessary do not react with an ionomer resin andincrease the viscosity of the mixture. Moreover, the compatibility canbe very effectively improved because the molecular ends of the blockcopolymer bond with the ionomer resin.

If the functional groups which react with an ionomer resin have too higha reactivity, the viscosity will decrease, whereas a reactivity which istoo low will lower the compatibility-improving effect. Hence, it ispreferable for the functional groups to have a moderate degree ofreactivity. Examples of such functional groups that may be used include,in order of decreasing preference, amino groups, acid anhydride groups,and epoxy groups. Amino groups are especially preferred.

The thermoplastic block copolymer has a melt index at 230° C. ofpreferably from 0.5 to 15 g/10 min, and more preferably from 1 to 7 g/10min. Outside of this range, problems such as weld lines, sink marks andshort shots may arise during injection molding.

The above components are compounded in a weight ratio a/b/c of 50 to80/50 to 20/4 to 20. In this ratio, the range of component (a) ispreferably at least 60%, more preferably at least 65%, and even morepreferably at least 70%, but preferably not more than 75%. The range ofcomponent (b) is preferably at least 23%, and more preferably at least25%, but preferably not more than 40%, and even more preferably not morethan 30%. The range of component (c) is preferably at least 5%, butpreferably not more than 15%, more preferably not more than 10%, andeven more preferably not more than 7%. Outside of this range, resiliencecannot be achieved and the compatibility worsens, as a result of whichlaminar separation may arise.

Component (e)

Component (e) is a fatty acid or fatty acid derivative having amolecular weight of at least 280 but not more than 1.500. Compared withthe base resin, this component has a very low molecular weight and, bysuitably adjusting the melt viscosity of the mixture, helps inparticular to improve the flow properties. Component (e) includes arelatively high content of acid groups (or derivatives), and is capableof suppressing an excessive loss in resilience.

The fatty acid or fatty acid derivative of component (e) has a molecularweight of at least 280, preferably at least 300, more preferably atleast 330, and even more preferably at least 360, but not more than1,500, preferably not more than 1,000, even more preferably not morethan 600, and most preferably not more than 500. If the molecular weightis too low, the heat resistance cannot be improved. On the other hand,if the molecular weight is too highs the flow properties cannot beimproved.

The fatty acid or fatty acid derivative of component (e) may be anunsaturated fatty acid (or derivative thereof) containing a double bondor triple bond on the alkyl moiety, or it may be a saturated fatty acid(or derivative thereof) in which the bonds on the alkyl moiety are allsingle bonds. It is recommended that the number of carbons on themolecule be preferably at least 18, more preferably at least 20, evenmore preferably at least 22, and most preferably at least 24, but notmore than 80, preferably not more than 60, more preferably not more than40, and even more preferably not more than 30. Too few carbons may makeit impossible to improve the heat resistance and may also make the acidgroup content so high as to diminish the flow-improving effect due tointeractions with acid groups present in the base resin. On the otherhand, too many carbons increases the molecular weight, as a result ofwhich a distinct flow-improving effect may not appear.

Specific examples of the fatty acid of component (e) include stearicacid, 12-hydroxystearic acid, behenic acid, oleic acid, linoleic acid,linolenic acid, arachidic acid and lignoceric acid. Of these, stearicacid, arachidic acid, behenic acid and lignoceric acid are preferred.Behenic acid is especially preferred.

The fatty acid derivative of component (e) is exemplified by metallicsoaps in which the proton on the acid group of the fatty acid has beenreplaced with a metal ion. Examples of the metal ion include Na⁺, Li⁺,Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Mn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Sn⁺⁺, Pb⁺⁺ andCo⁺⁺. Of these, Ca⁺⁺, Mg⁺⁺ and Zn⁺⁺ are especially preferred.

Specific examples of fatty acid derivatives that may be used ascomponent (e) include magnesium stearate, calcium stearate, zincstearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc12-hydroxystearate, magnesium arachidate, calcium arachidate, zincarachidate, magnesium behenate, calcium behenate, zinc behenate,magnesium lignocerate, calcium lignocerate and zinc lignocerate. Ofthese, magnesium stearate, calcium stearate, zinc stearate, magnesiumarachidate, calcium arachidate, zinc arachidate, magnesium behenate,calcium behenate, zinc behenate, magnesium lignocerate, calciumlignocerate and zinc lignocerate are preferred.

The amount of component (e) must be from 5 to 150 parts by weight per100 parts by weight of the base polymer, and is preferably at least 10parts by weight, and more preferably at least 15 parts by weight, butpreferably not more than 100 parts by weight, and more preferably notmore than 50 parts by weight, most preferably not more than 20 parts byweight. Resilience will not be achieved if component (e) is added in anamount below the above range. On the other hand, if component (e) isadded in an amount exceeding the above range, a large amount of gas willbe generated during injection molding, causing molding defects.

Component (f)

Illustrative examples of the metal ions in the basic inorganic metalcompound serving as component (f) include Li⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺,Zn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Knownbasic inorganic fillers containing these metal ions may be used as thebasic inorganic metal compound. Specific examples include magnesiumoxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodiumhydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithiumhydroxide and lithium carbonate. In particular, a hydroxide or amonoxide is recommended. Calcium hydroxide and magnesium oxide, whichhave a high reactivity with the base resin, are more preferred. Calciumhydroxide is especially preferred.

Component (f) must be included in an amount, per 100 parts by weight ofthe base polymer, of from 0.1 to 10 parts by weight. The amount ofcomponent (f) is preferably at least 0.5 part by weight, and morepreferably at least 1.0 part by weight, but not more than 8 parts byweight, and more preferably not more than 5 parts by weight. Ifcomponent (f) is added in an amount lower than the above range,resilience may not be achieved. On the other hand, if component (f) isadded in an amount greater than the above range, the flow propertiesduring injection molding will decrease, resulting in molding defects.

Other Components

In the practice of the invention, inorganic fillers which arenon-reactive with ionomer resins may additionally be added in an amountof from 10 to 30 parts by weight per 100 parts by weight of the basepolymer.

In such a case, to effectively improve the durability of the covercomposition, the average particle size of the inorganic filler ispreferably at least 0.01 μm, more preferably at least 0.05 μm, and evenmore preferably at least 0.1 μm, but preferably not more than 5 μm, morepreferably not more than 3 μm, and even more preferably not more than 1μm.

To adjust the specific gravity of the cover composition, the specificgravity of the inorganic filler is preferably at least 2, and morepreferably at least 4, but preferably not more than 7, and morepreferably not more than 5.

The above-mentioned “non-reactivity with ionomer resins” is verified bythe absence of foaming or a large rise in viscosity when the inorganicfiller is melt-mixed with an ionomer resin. For example, when an amountof inorganic filler such as may be used in the invention is added to thetypical ionomer resin available under the trade name “Himilan 1605” andmixed at 200° C. for 5 minutes, if foaming does not arise and the meltindex after mixing is 1 or more, the inorganic filler may be regarded ashaving no reactivity with the ionomer resin.

Examples of such inorganic fillers include barium sulfate, titaniumdioxide and hard clay. Barium sulfate is especially preferred. The useof precipitated barium sulfate is preferred because of its particle sizestability.

The materials which may be used in the layers other than the aboveintermediate layer are not subject to any particular limitation andinclude, for example, thermoplastic or thermoset polyurethaneelastomers, polyester elastomers, ionomer resins, highly neutralizedionomer resins, polyolefin elastomers or mixtures thereof. Any one ofthese may be used alone or two or more types may be used in admixture.Especially preferred examples include ionomer resins and highlyneutralized ionomer resins.

The intermediate layer has a thickness per layer of typically at least0.5 mm, preferably at least 0.8 mm, and more preferably at least 1.0 mm,but not more than 1.2 mm, and preferably not more than 1.1 mm. If thethickness is too large, the resilience may decrease. If the coverthickness is too small, the durability of the ball may decrease.

The material used in at least one intermediate layer has a Shore Dhardness of preferably at least 45, more preferably at least 47, evenmore preferably at least 50, and most preferably at least 52, butpreferably not more than 65, more preferably not more than 63, even morepreferably not more than 60, and most preferably not more than 58. Ifthe intermediate layer material is too soft, the spin rate may increase,lowering the flight performance. On the other hand, if the intermediatelayer material is too hard, the feel on impact of the ball may worsen.

If the golf ball of the invention has a plurality of intermediatelayers, the above-described material must be used in at least one of theintermediate layers.

In the practice of the invention, the material used to form theoutermost layer serving as the golf ball cover may be any suitablematerial selected from among known materials insofar as the objects ofthe invention are attainable. Example include thermoplastic or thermosetpolyurethane elastomers, polyester elastomers, ionomer resins,polyolefin elastomers and mixtures thereof. These may be used singly oras mixtures of two or more thereof. Thermoplastic polyurethaneelastomers and ionomer resins are especially preferred.

Illustrative examples of thermoplastic polyurethane elastomers that maybe used for the above purpose include commercial products in which thediisocyanate is an aliphatic or aromatic compound, such as Pandex T7298,Pandex T7295, Pandex T7890, Pandex TR3080, Pandex T8295, Pandex T8290and Pandex T8260 (all manufactured by DIC Bayer Polymer, Ltd.).Illustrative examples of suitable commercial ionomer resins includeSurlyn 6320, Surlyn 8120 and Surlyn 9945 (all products of E.I. DuPont deNemours and Co., Inc.), and Himilan 1706, Himilan 1605, Himilan 1855,Himilan 1601, Himilan 1557, Himilan 1555, Himilan AM7331 and Himilan1601 (all products of DuPont-Mitsui Polychemicals Co., Ltd.).

Moreover, to improve the rebound and scuff resistance of the ball, it ispreferable to use a material formed of (A) and (B) below as the cover:

-   (A) a thermoplastic polyurethane material (including a thermoplastic    urethane elastomer), and-   (B) an isocyanate mixture of (b-1) an isocyanate compound having two    or more isocyanate groups as functional groups per molecule    dispersed in (b-2) a thermoplastic resin which is substantially    non-reactive with isocyanate.

The thermoplastic resin (b-2) is preferably a resin having a low waterabsorption and excellent compatibility with thermoplastic polyurethanematerials. Illustrative examples of such resins include polystyreneresins, polyvinyl chloride resins, ABS resins, polycarbonate resins andpolyester elastomers (e.g., polyether-ester block copolymers,polyester-ester block copolymers). From the standpoint of reboundresilience and strength, the use of a polyether-ester block copolymer isespecially preferred. Illustrative, non-limiting, examples of theisocyanate compound include aromatic diisocyanates such as4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and2,6-toluene diisocyanate; and aliphatic diisocyanates such ashexamethylene diisocyanate. From the standpoint of reactivity and worksafety, the use of 4,41-diphenylmethane diisocyanate is most preferred.

To ensure particularly suitable flow properties for injection moldingand improve moldability, it is preferable to adjust the melt indices ofthe intermediate layer material and the cover material. It isrecommended that the melt index (MI), as measured at a test temperatureof 190° C. and under a test load of 21.2 N (2.16 kgf) in accordance withJIS K-6760, be at least 0.5 dg/min, preferably at least 1 dg/min, morepreferably at least 1.5 dg/min, and even more preferably at least 2dg/min, but not more than 20 dg/min, preferably not more than 10 dg/min,more preferably not more than 5 dg/min, and even more preferably notmore than 3 dg/min. A melt index which is too high or too low maydramatically lower the processability.

The cover serving as the outermost layer has a thickness of generally atleast 0.5 mm, preferably at least 0.7 mm, and more preferably at least0.8 mm, but generally not more than 2.0 mm, preferably not more than 1.5mm, and more preferably not more than 1.2 mm. Too large a thickness maylower the rebound of the ball, whereas too small a cover thickness maylower the durability of the ball.

The Shore D hardness of the material used to form the cover serving asthe outermost layer is generally at least 30, preferably at least 35,more preferably at least 40, and most preferably at least 45, butpreferably not more than 60, more preferably not more than 58, even morepreferably not more than 56, and most preferably not more than 54. Ifthe cover is too soft, the spin rate of the ball on shots with a drivermay increase, lowering the flight performance. On the other hand, if thecover is too hard, the spin rate on approach shots may decrease and thefeel of the ball on impact may worsen.

As with the method of forming the cover in conventional golf balls, anyof various known methods such as injection molding or compressionmolding may be employed to form the above-described intermediate layerand the cover serving as the outermost layer in the inventive golf ball.Likewise, the intermediate layer and the cover may be easily formed bysuitable selection of the various molding conditions such as theinjection temperature and time from within the commonly used ranges. Forexample, use may be made of a process in which, depending on the type ofball, a prefabricated core is placed within a mold, following which thematerials for the intermediate layer and the cover are respectivelymixed and melted under applied heat, and successively injection-moldedaround the core. Alternatively, use may be made of a process in which apair of hemispherical half-cups are molded beforehand from the covermaterial of the invention, then placed around a spherical body composedof the core encased within the intermediate layer and molded underpressure at 120 to 170° C. for 1 to 5 minutes.

In the golf ball of the invention, the cover may have a plurality ofdimples formed on the surface thereof, in addition to which the covermay be administered various treatments such as surface preparation,stamping and painting. In particular, the ease of operation involved inadministering such surface treatment to a golf ball cover made of thecover material of the invention can be improved on account of the goodmoldability of the cover surface.

The volume occupancy VR (units: %) of the dimples on the surface of thegolf ball is defined as the ratio of the volume of the dimples on thegolf ball surface to the volume of a hypothetical golf ball withoutdimples. For shots taken with a driver (W#1), the VR value of theinventive golf ball is generally at least 0.66, preferably at least0.70, and more preferably at least 0.75, but not more than 0.85,preferably not more than 0.82, and more preferably not more than 0.79.If the VR value is too small, the ball may describe a high trajectorythat results in little roll and thus shortens the total distancetraveled by the ball. On the other hand, if the VR value is too large,the ball may describe a flattened trajectory that fails to deliver thedesired carry and consequently shortens the total distance of travel.

The golf ball of the invention, which can be manufactured so as toconform with the Rules of Golf for competitive play, may be produced toa ball diameter which is not less than 42.67 mm, and preferably not morethan 43.00 mm, and to a weight which is not more than 45.93 g, andpreferably at least 44.6 g.

As explained above, the golf ball of the invention has a core, a coverserving as the outermost layer and, interposed therebetween, a thinintermediate layer of a desired hardness and a high quality. The ball isendowed with a good rebound, an excellent flight performance and anexcellent durability to cracking on repeated impact.

EXAMPLES

The following Examples of the invention and Comparative Examplesillustrate but do not limit the invention.

Examples 1 to 4, Comparative Examples 1 to 4

In each example, a solid core was fabricated by preparing a corecomposition from the common formulation shown in Table 1, then carryingout molding and vulcanization at a vulcanizing temperature and time of160° C. and 13 minutes. A first intermediate layer (envelope layer) wasmolded for this core from the formulation shown in Table 3, followingwhich a second intermediate layer and a cover as the outermost layerwere respectively injection-molded from the common formulations shown inTable 2. The first and second intermediate layers and the outermostcover layer were then successively placed and formed around the solidcore, thereby producing multi-piece solid golf balls. The physicalproperties and other characteristics of the resulting four-layer golfballs are shown in Table 3.

TABLE 1 Core formulation Polybutadiene rubber 100 Zinc oxide 16 Zincstearate 5.0 Zinc salt of pentachlorothiophenol 1.0 Antioxidant 0.1 Zincacrylate 39.0 Peroxide 3.0 Sulfur 0.1 Core properties Specific gravity1.17 Diameter (mm) 36.4 Weight (g) 29.4 Deflection (mm) 3.2 Note:Numbers in the table represent parts by weight.

Trade names for the chief materials appearing in Table 1 are as follows.

-   Polybutadiene rubber: Produced by JSR Corporation under the trade    name “BR 730”-   Zinc stearate: Produced by NOP Corporation under the trade name    “Zinc Stearate G”-   Antioxidant: 2,2-methylenebis(4-methyl-6-butylphenol), produced by    Ouchi Shinko Chemical is Industry Co., Ltd. under the trade name    “Nocrac NS-6”-   Peroxide: 40% Dilution in silica of    1,1-di(t-butylperoxy)cyclohexane, produced by NOF Corporation under    the trade name “Perhexa C40”

TABLE 2 Second Formulation Ionomer 68.75 intermediate (parts by weight)Olefinic thermoplastic elastomer 31.25 layer Behenic acid 18 Polyolefinpolyol 2 Calcium hydroxide 2.3 Specific gravity 0.93 Physical Resinsheet hardness (Shore D) 56 properties Thickness (mm) 1.2 Cover layerFormulation Thermoplastic Polyurethane A 50 (parts by weight)Thermoplastic Polyurethane B 50 Titanium oxide 4 Polyethylene wax 1.5Isocyanate mixture 15 Specific gravity 1.16 Thickness (mm) 1.0 Materialsurface hardness (Shore D) 48 Note: Numbers in the table represent partsby weight.

Trade names for the chief materials appearing in Table 2 are as follows.

-   Ionomer An ionomer resin which is a sodium ion-neutralized    ethylene-methacrylic acid copolymer produced by DuPont-Mitsui    Polychemicals Co., Ltd. under the trade name “Himilan 1605. ”-   Olefinic thermoplastic elastomer:    -   A hydrogenated polymer produced by JSR Corporation under the        trade name “DR 6100P.”-   Behenic acid: Produced by NOF Corporation under the trade name    “NAA-222S (powder).”-   Polyolefin polyol: A low-molecular-weight polyolefin polyol produced    by Mitsubishi Chemical Corporation under the trade name “Polytail    H.”-   Calcium hydroxide: Produced by Shiraishi Calcium Kaisha, Ltd. under    the trade name “CLS-B.”-   Thermoplastic: Polyurethane A:    -   MDI-PTMG type thermoplastic polyurethane produced by DIC Bayer        Polymer under the trade name “Pandex T-8290.”-   Thermoplastic Polyurethane B:    -   MDI-PTMG type thermoplastic polyurethane produced by DIC Bayer        Polymer under the trade name “Pandex T-8295.”-   Titanium oxide: Produced by Ishihara Sangyo Kaisha, Ltd. under the    trade name “Tipaque R550.”-   Polyethylene wax: Produced by Sanyo Chemical Industries, Ltd. under    the trade name “Sanwax 161P.”-   Isocyanate mixture: Crossnate EM30 (trade name), an isocyanate    masterbatch which is produced by Dainichi Seika Colour & Chemicals    Mfg. Co., Ltd., contains 30% of 4,4′-diphenylmethane diisocyanate    (measured concentration of amine reverse-titrated isocyanate    according to JIS K-1556, 5 to 10%), and in which the masterbatch    base resin is a polyester elastomer (Hytrel 4001, produced by    DuPont-Toray Co., Ltd.). The isocyanate mixture was mixed at the    time of injection molding.

TABLE 3 Example Comparative Example 1 2 3 4 1 2 3 4 First ComponentHimilan 1555 25 17.5 19 35 32 35 41 intermediate (a) Himilan 1557 35 3538 35 35 32 35 41 layer AM 7331 10 17.5 19 material Himilan 1601 35 (b)Primalloy N2800 25 25 20 25 25 30 25 15 (c) Terminal amino- 5 5 4 5 5 65 3 modified S-EB-C type thermoplastic block copolymer (d) Bariumsulfate 20 20 20 20 20 20 20 20 Magnesium stearate 1 1 1 1 1 1 1 1 (e)Behenic acid 17 17 17 17 17 17 17 (f) Calcium hydroxide 2.2 2.2 2.2 2.22.2 2.2 2.2 Shore D hardness 53 52 53 54 53 52 54 56 Specific gravity1.10 1.10 1.10 1.10 1.12 1.11 1.10 1.09 Thickness (mm) 0.95 0.95 0.950.95 0.95 0.95 0.95 0.95 First intermediate 38.3 38.3 38.3 38.3 38.338.3 38.3 38.3 layer-covered sphere (mm) Ball diameter (mm) 42.7 42.742.7 42.7 42.7 42.7 42.7 42.7 Ball deflection (mm) 2.44 2.43 2.45 2.432.49 2.48 2.45 2.42 Initial velocity (USGA 77.3 77.3 77.3 77.4 77.1 77.377.3 77.2 equivalent) (m/s) COR durability good good good good good NGNG good Melt Component (a) 62.4 68.9 68.8 76.8 52.3 52.3 52.3 52.3viscosity (Pa · s) Component (b) 81.2 81.2 81.2 81.2 81.2 81.2 81.2 61.2(Pa · s) Difference Viscosity 18.8 12.3 12.4 4.4 28.9 28.9 28.9 28.9(domain-matrix) The intermediate layer material formulation is shown inparts by weight.

Trade names and other details for the above materials are as follows.

-   Himilan 1555 (trade name):    -   An ionomer resin which is a sodium ion-neutralized        ethylene-methacrylic acid copolymer produced by DuPont-Mitsui        Polychemicals Co., Ltd.-   Himilan 1557 (trade name):    -   An ionomer resin which is a zinc ion-neutralized        ethylene-methacrylic acid copolymer produced by DuPont-Mitsui        Polychemicals Co., Ltd.-   AM 7331 (trade name): An ionomer resin which is a sodium    ion-neutralized ethylene-methacrylic acid-acrylic acid ester    copolymer produced by DuPont-Mitsui Polychemicals Co., Ltd.-   Himilan 1601 (trade name):    -   An ionomer resin which is a sodium ion-neutralized        ethylene-methacrylic acid copolymer produced by DuPont-Mitsui        Polychemicals Co., Ltd.-   Primalloy N2800 (trade name):    -   A thermoplastic polyester elastomer produced by Mitsubishi        Chemical Corporation.-   Terminal amino-modified S-EB-C type thermoplastic block copolymer:    -   Produced by JSR Corporation under the trade name “Dynaron        4630P.”-   Barium sulfate: Produced by Sakai Chemical Industry Co., Ltd. under    the trade name “Precipitated Barium Sulfate #300.”-   Magnesium stearate: Produced by NOF Corporation (Uji Sangyo) under    the trade name “Nissan Magnesium Stearate.”-   Behenic acid: Produced by NOF Corporation (Uji Sangyo) under the    trade name “NAA-222S (powder).”-   Calcium hydroxide: Produced by Shiraishi Calcium Kaisha, Ltd. under    the trade name “CLS-B.”    Melt Viscosity Measurement

Measured in accordance with JIS K-7199 using a capillary viscometer(Capillograph 1C, manufactured by Toyo Seiki Seisaku-Sho, Ltd.). Themeasurement conditions were as follows: temperature, 240° C.; capillary,L/D=10.0/1.0 mm. The melt viscosity (Pa·s) of specimens was measured ata shear rate of 6,000 sec⁻¹ at a piston speed setting of 500 mm/min.

Cover Resin Hardness

The Shore D hardness measured in accordance with ASTM D-2240.

Ball Deflection (mm)

The amount of deformation (mm) by the golf ball when compressed under afinal load of 1,275 N (130 kgf) from an initial load state of 98 N (10kgf) was determined.

Initial Velocity of Ball (m/s)

The initial velocity was measured using an initial velocity measuringapparatus of the same type as that of the official golf ballregulating-body—R&A (USGA), and in accordance with R&A (USGA) rules.

Durability to Repeated Impact

The golf ball was repeatedly struck at a head speed (HS) of 45 m/s, andthe durability to repeated impact was rated as the number of shots thathad been taken with the ball when the rebound underwent consecutivedecreases of 3%. Each value shown in the table is the average number ofshots for three balls.

-   -   Good: Did not exhibit cracking even after 200 or more shots.    -   Fair: Some balls exhibited cracking after less than 200 shots.    -   NG: Exhibited cracking after less than 200 shots.        Extrusion

The reaction twin-screw extruder used for the examples of the inventionwas a co-rotating intermeshing twin-screw extruder (screw diameter, 32mm; L/D=30; main motor output, 7.5 kW; with vacuum vent) of the typecommonly used for material mixture.

As is apparent from the results in Table 3 above, in Comparative Example1, because components (e) and (f) were not added, the first Intermediatelayer had a low degree of neutralization, substantially lowering theresilience. In Comparative Examples 2 and 3, the difference of the maltviscosities between (a) and (b), (that is, (b)−(a)) was too large,resulting in laminar separation of the resins during high-speedinjection molding and thus substantially lowering the durability. InComparative Example 4, the amount of component (b) added was small,resulting in a lower resilience.

1. A golf ball comprising a core, an outermost cover layer and one ormore intermediate layer therebetween, wherein at least one intermediatelayer includes a mixture comprising: 100 parts by weight of a basepolymer composed of (a) a metal ion neutralization product of anolefin-unsaturated carboxylic acid random copolymer and/or a metal ionneutralization product of an olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester random copolymer, (b) athermoplastic elastomer selected from the group consisting ofthermoplastic polyester elastomers, thermoplastic block copolymers andthermoplastic polyurethanes, and (c) a thermoplastic block copolymercontaining end blocks modified by a functional group having reactivitywith an ionomer resin, both end blocks being formed of differentcomonomers in a weight ratio a/b/c =50 to 80/50 to 20/4 to 20, (e) 5 to150 parts by weight of an organic acid of 18 to 40 carbons or aderivative thereof, and (f) 0.1 to 10 parts by weight of a metal ionsource selected from the group consisting of alkali metals, alkalineearth metals and transition metals; which mixture satisfies thecondition 0≦(b)-(a)≦25, where (b)-(a) is the difference between the meltviscosity of component (b) and the melt viscosity of component (a) asmeasured at a shear rate of 6,000 sec-1 and 240° C. in accordance withJIS K
 7199. 2. The golf ball of claim 1, wherein the mixtureadditionally comprises 10 to 30 parts by weight of an inorganic fillerwhich is non-reactive with the ionomer resin (d) per 100 parts by weightof the base polymer.
 3. The golf ball of claim 2, wherein component (b)is a thermoplastic polyester elastomer.
 4. The golf ball of claim 1,wherein the intermediate layer comprised of above components (a) to (f)has a thickness of at most 1.2 mm.
 5. The golf ball of claim 1, whereinthe material used in at least one intermediate layer has a Shore Dhardness of at least 45 but not more than 65.